This invention relates to black bodies or infrared radiation references.
More particularly, this invention relates to infrared radiation references formed of a plurality of discrete, thin, nestable members, preferably razor blades, which form part of an infrared radiation reference particularly suitable for calibration of certain instrumentation.
A black body is a radiator of uniform temperature whose radiant exitance in all parts of the spectrum is the maximum obtaining from any radiator at the same temperature. The black body is variously called a standard radiator or an ideal radiator.
The term black body denotes an ideal body which would, if it existed, absorb all and reflect none of the radiation falling upon it; its reflectivity would be zero and its absorptivity would be 100%. Such a body would, when illuminated, appear perfectly black and be invisible except as its outline might be revealed by the obscuring of objects beyond. The chief interest attached to such a body lies in the character of the radiation emitted by it when heated and the laws which govern the relations of the flux density and the spectral energy distribution of that radiation to the temperature.
The total emission of radiant energy from a black body takes place at a rate expressed by the Stefan-Boltzmann law, which states that the total electromagnetic emission of a black body is proportional to the fourth power of its absolute temperature; while its spectral energy distribution is described by Wien's laws, or more accurately by Planck's equation, as well as by a number of other empirical laws and formulae. Planck's formula indicates that a black body, which has a temperature between 50 Kelvin and 3000 Kelvin will emit electromagnetic radiation principally in the infrared region. This temperature range encompasses the temperatures at which most non-nuclear physical phenomena occur.
The nearest approach to the ideal black body, experimentally, is not a sooty surface, as might be supposed, but an almost completely closed cavity in an opaque body, such as a jug. The laboratory type is usually a somewhat elongated, hollow, metal cylinder, blackened inside, and completely closed except for a narrow slit in one end. When such an enclosure is heated, the radiation escaping through the opening closely resembles the ideal black body radiation; while light or other radiation entering by the opening is almost completely trapped by multiple reflection from the walls, so that the opening usually appears intensely black. For this reason, black body or "Planckian" radiation is also often called "cavity radiation."
Black bodies or infrared radiation references are necessary for the calibration of radiation pyrometers, satellite spectrometers, and have applications in the general area of radiation thermometry. Since accuracy is vital, the emissivity of these black body references must be known. Generally, a black body reference consists of some type of isothermal cavity or cavity array that is fabricated out of a relatively emissive material. With the appropriate cavity geometry and a sufficiently emissive cavity surface, the reference's emissivity will approach unity and, thus, be most useful because its calculated performance estimate is most correct. It must be emphasized that an accurate experimental determination of emissivity is not possible, and experimentors must rely on appropriate implementations of cavity structures that are amenable to analysis.
The most popular geometries for black body references are cones where the aperture dimension (open end diameter) is very much less than the axial length of the cone; or pierced spheres, where the aperture diameter is very much less than the sphere's diameter. These structures are generally large as compared with the aperture dimensions. Since the cavity surface must be isothermal in a precision reference, temperature controlling means and substantial amounts of thermal insulation add to the bulk of these laboratory references.
There is a class of more portable black body references that rely on highly emissive surfaces rather than large cavities. These surfaces may be an array of cylinders, honeycomb, pyramids, or other similar stuctures that facilitate multiple reflections. Not all of these geometric shapes have been treated analytically with respect to calculated emissivities, but they may be approximated by equivalent cylinders and cones to estimate emissivity in many cases. For a review of the state of the art of infrared references, see W. I. Wolfe and G. J. Zissis, Eds., "The Infrared Handbook", The Infrared Information and Analysis (IRIA) Center, Environmental Research Institute of Michigan, 1978, Chapter 2.
There is an emissive surface geometry, however, that is amenable to analysis. This is a parallel "V"-groove block as described in E. W. Treuenfels, "Emissivity of Isothermal Cavities", JOSA, Vol. 53, No. 10, October, 1963. pp. 1162-1171. Although relatively easy to analyze, the analysis assumes vanishingly small (actually zero) radii of curvature at the peaks and valleys of the triangular segments. Realistically, one cannot machine out of bulk metal a series of "V"-grooves with sufficiently sharp troughs and apexes. There will, in fact, be reflecting bands in the machined block at each trough and apex that reduce the emissivity (by virtue of the reflection) in an unpredictable (or non-analyzable) manner.